The present invention relates to a method and an apparatus for separating various mixtures of components utilizing the differences inn their mass, utilizing a flow, such as a fluid flow having these components suspended therein. More particularly, the present invention is directed to the mechanical separation of gases and gases having suspended therein solid components.
Isotope separation is presently a necessary process for the enrichment of fissionable fuels for most kinds of nuclear fission reactors, but consumes undesirably large amounts of energy and requires enormous capital investment in respect of process equipment and facilities. For example, conventional separation by gaseous diffusion techniques may consume about 2500 kilowatt hours per separative work unit (KWh/SWU) or more, and may require a complex and massive array of facilities with an amortized capital cost of, for example, over $250. per separative work unit per year.
However, the work of isotope separation has not been done efficiently by conventional separation techniques and apparatus. For example, with reference to the limiting factor of the thermodynamic entropy change in respect of different molecular species, the previously referred to processing energy ratio of 2500 kwh/SWU is more than seven orders of magnitude larger than the energy need for reversing the entropy increment resulting from the mixing at room temperature, of the different atomic weight components of the naturally occurring uranium isotope mixture of U.sup.235 F.sub.6 and U.sup.238 F.sub.6. Accordingly, the potential for substantially improving separative efficiency is high, and substantial research effort, governmental as well as industrial, has been devoted to the improvement of separation systems and techniques. The largest portion of this research effort has been directed to mechanical methods of separation such as those employing gaseous diffusion, centrifugation or curved-jets.
Also, the application of gas centrifugation techniques have been considered with respect to fuel cells, fuel cells mode electric power generation, refining of metals, etc. whose operations are based upon the use of air and/or oxygen enriched air. It is recognized that the efficiency of such operations can be improved by the practicability of using, instead of air, oxygen extracted from the air by means of centrifugation, as the fuel oxidizer.
It is further recognized that the centrifugation techniques are also applicable where the preferred substance for the process is other than oxygen, such as in the extraction of hydrogen from the steam-catalytic processing of coal at moderately high temperatures so as to produce a mixture of steam, hydrogen and carbon oxides wherein the latter can be eliminated by centrifugation.
Centrifugal separation techniques have long been known (e.g., U.S. Pat. Nos. 1,337,774 and 1,508,405) and have found utilization in applications such as separating solids from liquids, oxygen from air, and hydrogen from oil refinery gas. Basic techniques for centrifugal separation of gaseous isotopes were developed during the Manhattan Project (e.g., U.S. Pat. No. 2,536,423), with the development of systems utilizing a counter flow produced through application of a thermal gradient, and the provision of multiple, coaxial moving walls, being more recent events (e.g., U.S. Pat. Nos. 2,876,949 and 3,915,673). Such centrifugal separation systems may employ a spinning chamber surrounded by a vacuum, so that a mixture introduced into the center of the rotating chamber along its axis will tend to be separated into its component parts and such that a higher molecular weight (depleted) stream and a lower molecular weight (enriched) stream may be appropriately withdrawn (e.g., at opposite axial ends of the chamber) by the differential effect of centrifugal work performed on components having different molecular weights.
British Pat. No. 733,786 (Waagner-Biro) discloses a whirling chamber dust cleaning method wherein axially extending outlet ports reach only slightly into the whirling chamber apparently to avoid secondary current flows. Contrary thereto the invention uses secondary current flows to increase the cleaning efficiency as will be explained in detail below. The British Pat. No. 733,786 provides tangential exit ports adjacent the ends of the chamber for the removal of the fine dust. These exit ports are necessary because secondary currents are suppressed.
U.S. Pat. No. 474,490 (Walter) shows a whirling chamber with tangential influx of the flowing, contaminated medium and axial withdrawal of the gas. The axially extending exit pipes are axially adjustable in the extent to which these pipes reach into the chamber. However, the entire gas volume must exit through the axial pipes so that no advantage is taken of the formation of secondary current flows.
There are various disadvantages in respect of prior art gas centrifuges. In this connection, the massive moving wall of a gas centrifuge is at the largest radial distance, resulting in a large moment of inertia which can be a dangerous feature in such processing equipment. However, the most serious drawback of conventional gas centrifuge systems are limitations on the rate at which material may be processed by the equipment. Centrifugal separation of the constituent population is pressure diffusion limited, and this factor limits gas centrifuge throughput. The diffusion time necessary to traverse the radial distances used to provide high wall speed (as well as the axial distance which may be about ten times the radial dimension for in situ cascade centrifuge) is relatively long. This constraint severely limits centrifuge throughput.
In centrifugal systems, the gases co-rotate with the bowl at very high speed, so that there is little fluid dynamic problem with mixing caused by differential wall-gas velocity interaction. The separative flow is diffusion limited, and for a given periphery speed sustainable by a particular strength of material forming the bowl, the smaller the radius, the larger the centrifugal force to drive the pressure-diffusion. However, the high speeds which are necessary present certain difficulties. For example, at a typical radius of 10 cm with a typical wall-speed of 400 m/s, the bowl is already spinning at 38,000 rpm. This combination of very high periphery speed at relatively low throughput is a key bottleneck which drives up the capital cost. Also the weight of the spinning bowl is mostly concentrated at the wall of maximum radius, which gives rise to a large moment of inertia, and at high speeds, becomes a heavy burden to the rotating shaft to undergo sudden dislocation, such as earthquake or the breaking of a neighbouring bowl. Safety factors can translate quickly into high costs of capital as well as maintenance.
U.S. Pat. No. 4,193,775 (Wang) discloses turbo-separative methods and apparatus for separation of mixtures of gaseous materials having different molecular weights in which differential sedimentation velocities of the components to be separated are established in a laminar boundary layer of the gas adjacent a hydraulically smooth, porous blade surface, and in which a predetermined amount of the boundary layer flow is conducted through the porous surface to stabilize the boundary layer and to provide a higher density gas fraction.